Polyolefins, such as polypropylene and polyethylene have been used in many applications, for example in packaging applications. To form the package, it is often necessary to obtain a seal between two different parts formed of the same polyolefin. This may be clone using adhesives, but is more commonly done by applying heat to the surfaces to be joined to soften or melt them while applying pressure to form a single piece of thermoplastic. This operation is called heat-sealing, and is commonly used to join thermoplastic parts.
Efforts to improve heat sealing are a continuous focus of improvements.